As far as I know, this is not a programming bug. This is bad design resulting in user error. It is safe to delete sideblogs if you do it the correct (non-intuitive) way.
Source: I have successfully deleted multiple sideblogs over the years. And I have unfortunately heard of many people accidentally deleting their whole accounts over the years too. This Tumblr help page corroborates my experience and outlines the proper way to delete a sideblog. (Note the difference between the headers: “Deleting Your Account” vs. “Deleting a Secondary Blog or Leaving a Group Blog”)
When you go into the settings of a sideblog, there’s a button at the bottom that says “Delete [sideblog url]”
Of course, people think this button will delete only the sideblog they want to delete. But the thing is, this button shows up at the bottom of every blog in the settings page, and the url displayed will match whatever sideblog you’re looking at. It will still delete your entire account, as intended by staff.
What you need to actually do:
When you want to delete a sideblog, you must go to the members tab of that blog and hit “Delete this blog”
https://www.tumblr.com/blog/[sideblog url]/members
Sideblogs are meant to have the potential to be shared by multiple users. So the thinking is, if you are one of 5 members of a blog and you want to leave, you go into the members tab and leave. The 4 other people still share the blog without you.
That same line of thought applies when you are the only user. When you are the only owner of a sideblog, you must ‘leave’ the ‘group’ from the members page to delete it. Deleting any blog from the settings page will delete your entire account.
If you’re still unsure, you can follow these steps:
Create a throwaway account. Not another sideblog, but an entirely new account from scratch with a new email.
Add this throwaway account as a member to the sideblog (done on the same members page, can be seen in the screenshot above as “Invite to this blog”)
Once your throwaway is a member of the sideblog, you can leave the sideblog from your main. The throwaway should become admin of that sideblog by default once you leave.
Now you can test out what it’s like to properly delete a sideblog from a throwaway account. IIRC, you will be prompted to type the url of the sideblog along with your login info (email and password) to confirm.
